The Elven Ritual

(Just before this, in the room where the lost spirit boy was looking for his mother, you need to have searched the chambers to the sides, in a sarcophagus in one of the chambers is a tablet, read the tablet before attempting, otherwise you won't get the option to complete the ritual, when you kneel to pray, the water will boil and you will be attacked by a pair of shades. This was on PS3 that I encountered the issue with needing the tablet before the ceremony, may be different on PC or Xbox)

At the only location given on the map provided, you'll find an interesting puzzle. A gigantic locked door stands nearby, and the only way to open it and get to what lay on the other side of the door is to complete this elaborate, lost-to-history elven ritual. While you can get hints as to how this ritual is played out by exploring the ruins, you don't even need to find anything to get the ritual going. After all, you have our guide, and thus the exact order in which moves must be executed. So, head to location A on the map, descend the stairs towards the locked door, and do things in the order discussed here. For starters, examine the fountain in the middle of the room, and when prompted, opt to take the jug out of the fountain, which should now be filled with water. You can then leave the fountain and head for the nearby altar, along one of the walls. Examine the altar.

Once you've examined the altar, you'll be prompted on what to do next. Put the jug full of water on the altar, then kneel down and pray. While kneeling, take the jug and take a single sip from it. Then, leave the altar with the jug in hand, and go back to the fountain. Opt to dump the remaining water back into the fountain. If you followed our instructions to the tee, your actions will have unlocked the large door nearby, granting you access to a room full of goods and monsters alike. Successfully completing the ritual ends this brief side quest, though the room you now have access to is an important step in the side quest known as The Mage's Treasure. You must complete this side quest before completing the Mage's Treasure. In other words, you can't complete The Mage's Treasure without doing this side quest!